Fuel cell including a single sheet of a polymer electrolyte membrane (PEM), the PEM being divided into regions of varying electrical and ionic conductivity

ABSTRACT

A fuel cell is provided including a single sheet film or ribbon of any  leh of a Polymer Electrolyte Membrane (&#34;PEM&#34;) wherein the PEM is divided into regions or zones of varying electrical conductivity and ionic conductivity, allowing the elimination of endplates and/or a bipolar configuration. In this non bipolar arrangement, means are provided for distributing fuel to only one side of the PEM and oxidant to only the other side of the PEM so as to prevent mixing of the fuel and oxidant.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto us of any royalty thereon.

CONTINUATION

This application is a continuation of United States Patent and TrademarkOffice application Ser. No. 08/320,611, entitled, "Fuel Cell Including aSingle Sheet of a Polymer Electrolyte Membrane ("PEM"), the PEM BeingDivided into Regions of Varying Electrical and Ionic Conductivity,"filed on Oct. 6, 1994, by the same inventors herein, and now abandoned.This continuation is being filed under 35 USC §120 and 37 CFR §1.53.

FIELD OF INVENTION

The invention relates in general to fuel cells and in particular, tofuel cells including a single sheet of a polymer electrolyte membrane("PEM") wherein the PEM is divided into regions of varying electricaland ionic conductivity.

BACKGROUND OF THE INVENTION

Fuel cell power systems are bound to be one of the more important futurealternative power sources and energy conversion devices for portablepower packs since they are lightweight and have low noise and thermalsignatures. Fuel cells are readily adapted for various applicationssince unlike other electrochemical sources, which can operate on onlyone type of fuel, fuel cells may be operated with a number of differentfuels. By silently converting chemical energy of various fuels directlyinto electricity, water, and heat, fuel cells provide clean,nonpolluting energy, with only water and heat as byproducts. Forelectric vehicles, man-portable power systems or remote power systemswhere lightweight fuel cells are required, there has been significantand renewed interest in developing fuel cells using polymer electrolytemembranes, PEMs. Here, a thin PEM simultaneously acts as (a) the protonconducting electrolyte; (b) separator which prevents the air or oxygenon the cathode side from contacting the hydrogen or other fuel at theanode and (c) catalyst support. Use of a single, thin PEM instead of anelectrolyte made of either aqueous solutions, molten salts or solidoxides liberates this type of fuel cell technology from the storage andmaintenance problems facing corrosive and/or high temperatureelectrolytes. PEM based fuel cells are extremely attractive because allcomponents are solid, noncorrosive, and operational temperatures aregenerally below 100° C.

Fuel cells using PEMs are expected to have long shelf life on opencircuit stand and should be capable of starting up and operating atrelatively low temperatures (<100° C.) since PEMs do not require highertemperatures to achieve good ionic conductivity. Polymer based protonconductors provide both the medium for bulk proton transport and theinterfacial environment for the fuel oxidation and reduction reactions.

Typical PEMs, approximately 50-200 micron thick, are based on polymersthat have PTFE-like backbones and that have fluorocarbon-based sulfonicacid side chains (--CF₂ --SO₃ H) covalently attached as pendant groups.The acidic sulfonic acid terminal group, --SO₃, is highly ionized inwater, with the proton being the mobile replaceable ion, while thesulfonate group is fixed in the polymeric matrix. These SO₃ H groupsprovide cation exchange capacity and ionic conduction for typical PEMssuch as Dupont's Nafion series, Dow's lower molecular weight analog(XUS), Membrane C by Chlorine Engineers, Japan, and Aciplex S membranefrom Asahi Chemical Industry Company Ltd. of Japan.

Since fuel cells using hydrogen, methanol, or other liquid fuelsgenerally provide operating voltages below 1.0 volt, and since practicalfuel cells must usually provide from at least several volts up to 100volts or more, the usual approach is to construct many separate pairs ofcells in a multicell fuel cell stack using a bipolar design. In thisdesign, a number of individual cells (each cell having its own PEM) areelectronically connected in series by abutting the anode currentcollector of one cell with the cathode current collector of its nearestneighbor in the stack. Each individual cell generally includes an anode,a cathode, a common electrolyte (PEM) and a fuel and oxidant source.Both fuel and oxidant are introduced through manifolds to theirrespective chambers. The dilemma with sealing and manifolding this piledor layered design is that individual cells each with its own PEM must benecessarily held together with a clamping frame and tie endplates. PEMfuel cell stacks must also include the required gas diffusers that areusually interspersed between active single cells each containing its owndiscrete PEM. These gas diffusers provide only limited transport ofoxygen (either pure oxygen or by using air). Increasing gas transportrequires either pressurization of air or forced convection. In addition,since in order for a fuel cell to operate, hydrogen gas (fuel) andoxygen gas must each be kept to its own side of each membrane,appropriate hydrogen and oxygen delivery systems are used to feed theproper gas to the correct side of each individual cell and eachmembrane. All of this adds bulk, weight, cost and complexity to thestack. This design also suffers from heat management problems sincecells within the bipolar arrangement, i.e. those far from the stackends, are not sufficiently cooled by either air or the coolant.

An alternate configuration would be a side by side series connection inwhich a number of individual cells each with its own discrete PEM sheetor film are placed next to each other in a horizontal, planar, or woundconfiguration. This arrangement eliminates the bulky tie rods, andconnectors, and minimizes heat transfer problems since the cells areconnected together in a side by side arrangement. However, in order toconnect the individual cells with their discrete PEM in series (in sucha manner that the cathode of cell 1 is connected with the anode of cell2), an electronic conductor must pass through the plane of the attachedcells and connect to adjacent cells. An example of this is U.S. Pat. No.5,190,834, by Kendall that discloses and claims a composite membranecomprising a domain of conductive and another domain of electricallyinsulating materials connected by a feedthrough that traverses the planeof the membrane and connects from the bottom of one cell to the top ofthe adjacent cell. The major flaw in the Kendall design is that itrequires an insulating elastomer or other type of material to separateeach individual cell from its nearest neighbor. This arrangement is notsatisfactory in the case of fuel cells, since this adhesive bond betweenPEM cells would not be impermeable to hydrogen gas, methanol or otherfuel, over the life of the cell. Furthermore, the adhesive connectorwould be unable to allow for expansion and contraction of individualcells (since the PEM expands and contracts) as a function of temperatureand water content. In addition, since there is an adhesive or connectorbetween cells, the Kendall cell assembly is not completely sealed andthe assembly process for sealing cells together is labor intensive.

SUMMARY OF THE INVENTION

The general object of this invention is to provide a design for a fuelcell that consumes gaseous or liquid fuels that will overcome theaforementioned difficulties. A more particular object of the inventionis to provide a novel nonbipolar design for a fuel cell, capacitor orsensor that eliminates the conventional bipolar design by changing to aside by side series configuration of individual PEM cells fabricatedfrom a single uncut sheet, film or roll of PEM. A particular object ofthe invention is to provide a design for a fuel cell that utilizes asingle sheet or ribbon of PEM that is subdivided chemically into zonesor regions so that a number of individual cells can be made from asingle sheet or ribbon of PEM. This differs from ideas where electrodesare embedded in a matrix for here the PEM itself acts as the matrix orsupport.

It has now been found that the aforementioned objects can be attained byproviding a single sheet of PEM of any length that is subdivided intoregions, zones bands or sections of varying electrical and ionicconductivity and a means for distributing fuel to one side of this PEMand oxidant to the other side of this PEM in such a manner as to preventmixing of the fuel and the oxidant. In this invention, the zones orregions are not taped or glued together but rather are made from thesame PEM. In this way the problems with sealing the joints between thedifferent regions is alleviated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a side by side series configuration ofindividual PEM cells made from a single sheet of PEM according to theinvention.

FIG. 2 is a side view which depicts the PEM being sealed.

FIG. 3 depicts an alternate configuration of the present invention byhaving the PEM in a winding or ribbon configuration.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a single sheet of PEM 10, that can be solutioncast or melt extruded is deliberately separated and divided intoregions, zones or bands, which could include a plurality of regionswhich can be made, in varying degrees, to be conductive ornon-conductive so that conductivity is either enhanced or diminished,that traverse the membrane thickness, having the designation and repeatpattern of 12, 14, 16, 14-12', 14', 16'-14'-12", 14", 16", 14" where the' or " refers to a repeat of the pattern, which may also be referred toas an ABC-ABC-ABC repeat pattern. Each of said plurality of regions ofvarying electrical and ionic conductivity having varying electrical andionic properties, further comprising electrical conductivity, ionicconductivity, electrical non-conductivity and ionic non-conductivity.

In FIG. 1, regions 12, 12' and 12" are taken to represent a plurality ofregions of PEM, being an "A", or first, region in the repeat patternABC-ABC-ABC, that are ionically conducting but electricallynonconductive. This is characteristic of untreated PEM. Regions 14, 14',and 14" are taken to represent a plurality of regions, being a "B", orsecond, or middle separation, region in the repeat pattern ABC-ABC-ABC,having sufficiently low ionic conductivity so that hydrogen ions,commonly called protons, cannot travel diagonally across said firstregion to said second region and a plurality of like regions 12, 12',and 12" have little or no electrical conductivity. Regions 14, 14' and14" may also be designed to be wide enough to deter hydrogen ions fromtravelling diagonally across said first region to said second region, inlieu of having a given level of ionic conductivity. This region or zoneacts as the electrical spacer. Regions 16, 16', and 16", being a "C"region in the repeat pattern ABC-ABC-ABC, is deliberately made on saidPEM sheet 10 to be sufficiently electrically conductive to allow ease ofelectron transfer from the anode side of region 12 to the cathode sideof region 12', or from one cell to the next cell.

All of these regions i.e., 12, 14 and 16, traverse the thickness of themembrane. This repeat pattern can be designed into a single, continuoussheet of any length of PEM by processing the membrane so that differentsections of the PEM have different electrical properties. Regions 12,12', and 12" are the untreated, as formed, regions of the PEM. Regions14, 14', and 14" are regions that are deliberately made to be bothlonically nonconducting and electrically insulating. Regions 14, 14' and14", that electrically isolate regions 12, 12' or 12" from 16, 16', or16", respectively, can be created subsequently to film formation forexample, either by chemically modifying the PEM to reduce its ionicconductivity, or by exposing the desired regions of the PEM to a CF₄ gasplasma that substantially reduces the ionic conductivity in this regionof the PEM. Regions 16, 16', and 16" can be created in the PEM byblending or mixing the PEM with conductive polymers, or by irradiatingit with ion beams to implant species that will increase the electricalconductivity.

The fuel cell of the present invention is sealed along an upper edge 31and a lower edge 32 of the PEM 10 to prevent gases or liquids frommixing with each other so that the fuel, either hydrogen or any othersuitable fuel, comes in contact with one side while the oxidant, oxygen,only comes in contact with the other side of the PEM. The means fordistributing a fuel to only one side of said PEM sheet and distributingan oxidant only to the other side of said PEM sheet must prevent mixingof said fuel and said oxidant.

FIG. 2 illustrates a side view of the PEM 10 being sealed. Referring nowto FIG. 2, a seal 36 seals said upper edge 31 of the PEM 10. Similarly,a seal 37 seals said lower edge 32 of the PEM 10.

In the foregoing embodiment one could also conceivably start with a PEMor other material that is completely insulating as for example, inregions 14, 14' and 14". By selectively and deliberately altering thePEM so that certain desired regions will have characteristics of regions12, 12' and 12 i.e., ionically conducting and electrically insulatingand regions 16, 16' and 16" (electrically conducting) the same effectscan be achieved.

The fuel and oxidant can now be easily distributed by sealing along theedge or periphery of the PEM so that the fuel comes in contact with onlyone side while the oxidant only comes in contact with the other side ofthe PEM. A solid DC-DC converter can be used to raise the stack voltageto a desired system output voltage.

More particularly, according to the invention, in an operational fuelcell using this design, hydrogen gas enters on the anode side of the PEM10, and passes through gas diffusion electrodes 26, 26' and 26" that arein contact with the anode catalyst 18, 18' or 18". The hydrogen thencomes in contact with anode catalyst 18, 18' and 18" and is dissociatedinto protons and electrons according to the equation:

    H.sub.2 →2H.sup.+ +2e.sup.-

The formed protons then travel through the PEM regions (12, 12' or 12")towards the cathode side. Electrons, formed by the anode reaction above,travel through the electrical connectors 20, 20' and 20" that areattached to the gas diffusion electrode 26, 26' and 26", travel throughconductive zones 16, 16' or 16" in the PEM sheet 10, and pass throughthe electrical connectors 22, 22' and 22" on the cathode side, to thegas diffusion electrodes on the cathode side 28, 28' and 28". On thecathode side, oxygen gas passes through the gas diffusion layer 28, 28'and 28" and is electrochemically reduced on the cathode catalyst 24, 24'and 24" to water by the reaction:

    2e.sup.- +1/2O.sub.2 +2H.sup.+ →H.sub.2 O

Improved construction and cell arrangement can be made forelectrochemical devices such as batteries, fuel cells, sensors or thelike where a PEM is comprised of alternating regions that are ionicallyconductive and electrically insulating, regions that are both tonicallyand electrically insulating, and finally, regions that are electricallyconducting. Referring now to FIG. 3, the entire PEM sheet 40 assemblycan be wound in a ribbon type configuration around a central core 45where a gas or liquid contacts an interior side 48 of said PEM sheet 40contiguous with said central core 45 and another gas or liquid contactsan exterior side 49 of said PEM sheet 40.

FIG. 3 depicts the ribbon-winding configuration of the present inventionshowing said PEM sheet 40, or a plurality of similar PEM sheets, beingwound around said central core 45, said PEM sheet 40 having a pluralityof regions including a first region of PEM 41 being ionically conductingbut electrically insulating, a second region of PEM 42 having very poorprotonic conductivity and little electrical conductivity, and a thirdregion 43 being highly electrically conductive. Said PEM sheet 40 havingan upper edge 46 and a lower edge 47. The PEM membranes are sealed atthe upper and lower edges, 46 and 47, respectively, to prevent gases orliquids from mixing with each other. While FIG. 3 depicts a spiral typeof winding configuration, the present invention also encompasses otherwinding or ribbon-type configurations.

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and described for obviousmodification will occur to a person skilled in the art.

What is claimed is:
 1. A fuel cell including a single sheet of polymerelectrolyte membrane (PEM), comprising:said PEM sheet being divided bychemical treatments into a plurality of regions; each of said pluralityof regions having varying electrical and ionic conductivity propertiesfurther comprising electrical conductivity, ionic conductivity,electrical non-conductivity, ionic non-conductivity and combinationsthereof; said PEM sheet having a means for distributing a fuel to onlyone side of said PEM sheet and distributing an oxidant only to the otherside of said PEM sheet in such manner as to prevent mixing of said fueland said oxidant; said plurality of regions of the PEM sheet are furtherdivided into at least three regions with a designation and repeatpattern of ABC-ABC-ABC; said A region of the plurality of regions beingionically conductive but electrically non-conductive; said C region ofthe plurality of regions being electrically conductive but ionicallynon-conductive and said B region of the plurality of regions havingionic conductivity lower than said A region, and having electricalconductivity lower than said C region.
 2. A fuel cell, as recited inclaim 1, wherein said means for distributing said fuel to the one sideof said PEM sheet and distributing said oxidant to the other side ofsaid PEM sheet prevents mixing of said fuel and said oxidant by sealingsaid PEM sheet along an upper edge and a lower edge of said PEM sheet sothat said fuel only comes in contact with the one side of said PEM sheetwhile said oxidant only comes in contact with the other side of said PEMsheet.
 3. A fuel cell, as recited in claim 1, comprising:a plurality ofPEM sheets in a side by side series configuration being wound around acentral core; and each of said plurality of PEM sheets having an upperedge, a lower edge, an interior side contiguous to said central core andan exterior side.